1. Field of the Invention
The present invention relates to a full-color light emitting device capable of emitting light beams of three or more colors in which three light emitting diode chips thereof having different light emission wavelengths are individually adjusted in light emission density, so that it can emit light beams of three or more colors, and more particularly to a full-color light emitting device with four leads in which three light emitting diode chips thereof having different light emission wavelengths can be individually controlled to realize emission of light beams of more diverse colors, while having a simplified connection structure, so that the light emitting device can be implemented even in the case in which a limited bonding area is provided.
2. Description of the Related Art
Light emitting devices, which use semiconductor light emitting elements, are configured by arranging, for example, a plurality of light emitting diodes (LEDs) as light emitting semiconductor elements on a panel. In such a case, each LED emits red, green or blue light in accordance with the kind of its compound semiconductor.
In the case of a light emitting device adapted to emit monochrome light, using LEDs as semiconductor light emitting elements, each LED constitutes one pixel. In the case of a light emitting device adapted to emit full-color light composed of the three primary colors, that is, red (R), green (G), and blue (B), using LEDs, each light emitting element thereof consists of three LEDs of red, green, and blue, that is, the three primary colors. In the latter case, each full-color light emitting element constitutes one pixel.
FIG. 1 illustrates a conventional full-color light emitting device for one light emitting element thereof. As shown in FIG. 1, first through third LEDs 14 to 16 are mounted on a main lead 11. The first LED 14 has a first electrode electrically connected to the main lead 11 in accordance with a die-bonding method, and a second electrode electrically connected to a first sub-lead 12 at its second electrode in accordance with a wire-bonding method. The second LED 15 has first and second electrodes respectively electrically connected to the first sub-lead 12 and a second sub-lead 13 in accordance with a wire-bonding method. The third LED 16 has a first electrode electrically connected to the main lead 11 in accordance with a die-bonding method, and a second electrode electrically connected to the second sub-lead 13 in accordance with a wire-bonding method. The first and second electrodes of each LED may be an anode and a cathode, or vice versa, respectively.
In each of the first and third LEDs 14 and 16, one of its anode and cathode is arranged on the upper surface of its chip, whereas the remaining electrode is arranged on the lower surface of the chip. The electrode arranged on the lower chip surface is electrically connected to the main lead 11 in accordance with a die-bonding method, whereas the electrode arranged on the upper chip surface is electrically connected to an associated one of the first and second sub leads 12 and 13 in accordance with a wire-bonding method. On the other hand, in the case of the second LED 15, both electrodes thereof are arranged on the upper surface of its chip. In this case, the chip of the second LED 15 is mounted on the main lead 11 such that its lower surface is in contact with the main lead 11 via an insulating substrate. In this state, the electrodes on the upper chip surface are electrically connected to the first and second sub-leads 12 and 13 in accordance with a wire-bonding method, respectively.
The first LED 14 is an red (R) LED, the second LED 15 is a green (G) LED, and the third LED 16 is a blue (B) LED.
FIG. 2 illustrates an equivalent circuit of the full-color light emitting device implemented as shown in FIG. 1.
Now, operation of the full-color light emitting device will be described with reference to the equivalent circuit of FIG. 2. The color of light emitted from the light emitting device can be adjusted by controlling respective voltages applied to the three leads 11 to 13, thereby controlling respective operations of the first through third LEDs 14 to 16.
For instance, when a “+” voltage is applied to the main lead 11, and a “−” voltage is applied to the first sub-lead 12, the first LED 14 is activated. When the “+” voltage is applied to the main lead 11, and the “−” voltage is applied to the second sub-lead 12, the third LED 16 is activated. On the other hand, when the “+” voltage is applied to the first sub-lead 12, and the “−” voltage is applied to the second sub-lead 13, the second LED 15 is activated. When each LED of the full-color light emitting device is activated, it serves as a red, green or blue light source.
However, the above mentioned conventional full-color light emitting device has a high possibility of error generation because its operation condition is determined in accordance with the polarity of the control voltage applied to each of the first and second sub-lead 12 and 13.
Furthermore, the conventional full-color light emitting device has a complex electrical circuit configuration for implementation of full-color light emission, as shown in FIG. 2, because it uses a small number of lead frames.
In the above mentioned structure, there is also a problem in that it is difficult to configure a desired circuit where each of the LEDs 14 and 16, to which a die-bonding technique is to be applied, only has two or more wire bonding pads due to the substrate material of its chip.
FIG. 3 is a plan view illustrating another conventional full-color light emitting device with four leads. FIG. 4 illustrates an equivalent circuit of the full-color light emitting device shown in FIG. 3.
The full-color light emitting device shown in FIGS. 3 and 4 includes an R LED 35, a G LED 36, and a B LED 37 which have different light emission wavelengths, respectively. The R, G, and B LEDs 35 to 37 are bonded to a main lead frame 31 by means of an adhesive. The LEDs 35 to 37 are electrically connected to first through third sub-lead frames 32 to 34 for supply of electric power, respectively, while being electrically connected to the main lead frame 31 as a common electrode.
The electrical connection of each LED is achieved in accordance with a die-bonding method and a wire-bonding method using electrical connecting members (for example, conductive wires).
In this full-color light emitting device, as shown in FIG. 4, each of the three LEDs 35 to 37 is connected to the main lead frame 31 at one electrode thereof (anode) while being connected to an associated one of the first through third sub-lead frames 32 to 34 at the other electrode thereof (cathode). Each of the first through third LEDs 35 to 37 is turned on/off when the control voltage to be applied to an associated one of the first through third sub-lead frames 32 to 34 is switched on/off. Light beams emitted from the LEDs 35 to 37 in their ON state are mixed so that light of full color including red, green and blue, and mixed colors thereof is generated.
This conventional 4-lead full-color light emitting device can have a simple circuit configuration, as shown in FIG. 4. However, the main lead frame 31 must have a substantial area because the first through third LEDs 31 to 33 are commonly connected to the main lead frame 31 at their one-side ends. For this reason, where the full-color light emitting device has a limited size, an insufficient bonding area may be provided.
In other words, the above mentioned conventional 4-lead full-color light emitting device has a problem in that it cannot implement a full-color light emitting device including a main lead frame having a limited area.